A-level Biology TRANSLOCATION OF SUCROSE- mass flow hypothesis in plants and structure of the phloem

Learn how sugars are transported in plants. Translocation of organic substances, like sucrose, is transported to all cells in the plant through the phloem. Learn the structure of phloem tissues, companion cells and the sieve tube elements. Learn the source to sink model, mass flow hypothesis, translocation and the use of tracers and ringing experiments to investigate transport in plants.
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[@MissEstruchBiology]

Hello! I hope you found this helpful :D If you did, then head over to @MissEstruchBiology Instagram for more study tips, biology help, and daily quizzes instagram.com/missestruchbiology/?hl=en OR if you want to get your hands on some A-level NOTES READY MADE by me with key marking points, keywords, and essay links, make your way to missestruch.com/ (this will be available from July 2021 - Can't wait?? email [e-mail] to get your hands on a copy before then).

[@kunchaako3848]

i honestly dont think I could survive without your videos THANK U!!

[@MissEstruchBiology]

Aww thank you! I'm so pleased that you are finding them so helpful. Best of luck with your studies!

[@bcmiller2000]

I realize this is a bit dated, but info seems just as relevant and important. You're a fantastic teacher; I wish you were my teacher 50 years ago, you deserve some kudos!

[@RaspberryMalina190817]

I was a little confused on how sucrose was co-transported, and then you explained it very well! thank you so much

[@rosalina6677]

Thank you for this! I was following along with this video with my AQA textbook and everything your explaining is perfect. My teacher is missing things out, even though we're doing AQA. Your videos are helping lots!!

[@MissEstruchBiology]

Really please they are helping you so much, thank you for your comment! I have taught AQA for over 10 years now, so it is a specification I know well. Good luck!!

[@amzant7444]

I just wanted to say thank you so much!! I have my biology mock today and your video's on the more complicated topics have helped me so much

[@asmaqayum7391]

Amazing video thank you for all your help

[@user-uv7og9um9r]

Thank for your services

[@jy-gq6to]

Thank you! most helpful video I have watched all year

[@MissEstruchBiology]

Wow, thank you! So pleased it helped you.

[@alejandravillegas9478]

Thank you so much, i couldnt understand this topic and now i do!

[@MissEstruchBiology]

You're welcome 😊 So glad it's helped you to understand

[@yusrahndumbogani8653]

Very very good video loved this👍🏾

[@MissEstruchBiology]

Thank you. Really pleased you like it and I hope it helps with your studies.

[@daisylawrence7447]

Hi Miss Estruch, At 9:46 the slide says sucrose is used in respiration so decreases in the sink cell which means more sucrose enters the sink cell decreasing water potential. CGP just says: At the sink end, solutes are removed from the phloem to be used. This increases the water potential inside the sink cells, so water leaves the sink cells by osmosis. This lowers the hydrostatic pressure inside the sieve tubes at the sink end. is this CGP explanation wrong or too simplistic as your slides talk about the re-entering of sucrose into the sink cells after it has been used ?

[@marcusabston6365]

Thank you for a very straight forward explanation I'm zoology but I'd forgotten about this process. Thanks

[@MissEstruchBiology]

So glad you found my explanation clear 😁 Hope that helped with your zoology

[@marcusabston6365]

@@MissEstruchBiology yes I'm transitioning from collegiate level to high school level teaching. So I'm in the process of going over things I'm rusty in or was poorly taught. I have to go over PCR, DNA gels, etc. too because I never went through those labs which is sad and I have a Master of Science.

[@__-fl7pd]

Why is it active transport when sucrose moves from sieve tube into sink cells?

[@MissEstruchBiology]

Hi, The reason for this is because although the sink cells will be respiring, and therefore using up sugar, they still will contain a very high concentration of sucrose due to continual translocation occurring. Therefore, in order for sucrose to transport from the sieve tube element to the sink cell, active transport occurs as the sucrose is moving against the concentration gradient. I hope this helps.

[@aryanmalik1325]

Thnkyou mam for easily and connecting all the relative information 😀

[@MissEstruchBiology]

You're welcome 😊

[@ohyikes7104]

Hi - a quick question! If the sink has a low sucrose level due to it being used up by respiring cells, why does sucrose have to be actively transported across? Surely wouldn't it just diffuse? (Or be co-transported...)

[@ayoyimikaemmanuel3607]

Thank you so much😀

[@MissEstruchBiology]

you're welcome 😊 Hope it helps you.

[@patriciaM1]

Amazing Teaching

[@MissEstruchBiology]

😊😊 thank you

[@lolac5395]

hiya! I may have misunderstood but at 7:29 I feel like you said something different to what's written on the slide. So, you said that the H+ ions are transported via the co-transport molecule back *into the companion cell* whilst the sucrose moves into the sieve tube element But on the slide it says that the H+ ions move also *into the sieve tube element* alongside the sucrose. Was a bit confused on where the H+ ions transport to after being actively transported out of the companion cells?

[@MissEstruchBiology]

Hello, I can't see where it says its moving alongside on the side at the time. However, it may be the clarity of what I have written is unclear. It is two steps. Firstly, the H+ move down the concentration gradient from the companion cell to the sieve tube element, and then the H+ move back into the companion cell through the cotransporter, which simultaneously moves sucrose from the companion cell in the opposite direction. I hope that helps..

[@lolac5395]

@@MissEstruchBiology Yes i think it does! Thank you so much, this is a great video by the way, I learnt so much.

[@a9fc]

Working on a diploma and the course creator's 'e-learning' slides and robo-voiced script was rubbish. Thanks for this!

[@MissEstruchBiology]

Sorry to hear the subtitles weren't great for you. These are automatically generated by KZbin, and are not always perfect.

[@linguois1540]

why does sucrose travel via active transport into the sink cell at the end, when it is used up in respiration so shouldn't it just be diffusion along the gradient?

[@MissEstruchBiology]

hello, Good question and actually a lot of it would diffuse in due to the concentration gradient.

[@HaidarAli-ni3hf]

I don't understand Translocation 1. If the Sucrose is being co-transported from the companion cell then would it not end up in the cell wall not the sieve tube elements? I would really appreciate it if you could answer this Question

[@bhoomika7240]

Hi, thank you so much for all your videos. The AQA oxford textbook gets sucrose loading mechanism wrong (according to my teachers), so have you explained it like the textbook or the correct way?

[@user-xs9tf5re1c]

TYSM for ur incredibly useful vids as always. j a quick q, do we need to know the structure of xylem, the details about the movement of water in the root, and about the vascular bundle arrangement in the leaf, root, and the stem of a plant for AQA AS level ?

[@patriciaM1]

Thank you

[@MissEstruchBiology]

you're welcome!

[@lopsided4831]

Life saver

[@MissEstruchBiology]

Glad it helped 😊

[@samsephton5788]

Thanks again for the great video- life saver!! Does the co-transport occur when the sucrose and H2O is diffusing into the sink cell or is it just source to sieve tube element? Also when does the fructose get added to the glucose (from photosynthesis) to make the sucrose? Or have I got the wrong end of the stick? Thanks again for your help!!

[@melissacurtis7207]

sucrose concentration in the sink is lower than in the sieve tube elements so it's transported by diffusion and then active transport (when conc is lower in sieve tubes than sink). fructose + glucose happens in the cytoplasm of leaf cells after photosynthesis (bc sucrose has more energy than a monosaccharide) and then the sucrose is transported by mass transport to the other cells of the plant that aren't photosynthesising. HI SAM

[@samsephton5788]

@@melissacurtis7207 HAHAHA HI MEL

[@aeshakhan-by2bu]

7:35 is a bit vague? How does sucrose enter both companion cells and sieve cells?

[@noahblackwelll]

DIfferent videos are saying different things about transport of sucrose so im a bit confused. Is this correct? 1)- H+ ions actively transported to leaf tissue from companion cells 2) H+ diffuse back down gradient to companion cells through co transporter, with sucrose. 3) sucrose diffuses down conc gradient into sieve tube element (that was the snap revise explanation)

[@MissEstruchBiology]

yeah unfortunately the science isn't fully known and that's why it varies

[@noahblackwelll]

@@MissEstruchBiology ah right so any are ok in the exam?

[@hlovestxt8726]

what is the difference between apolastic and symplastic pathway

[@zuzannaopalka8800]

Hi, I wanted to ask if it’s necessary to know the apoplast and symplast pathways for the movement of water for AQA. it’s not in the spec and I can’t find any AQA videos on this. Thank you

[@MissEstruchBiology]

Hey not for AQA

[@newlau7604]

In translocation 3, why isn’t the water potential in the sieve tube element increase as the sucrose is actively transported into the sink cell?

[@MissEstruchBiology]

hello. The water potential does increase for that exact reason and then because the water potential is higher, water moves by osmosis into the sink cell and some into the xylem.

[@BakariHenryGideon]

Explicit and simple

[@MissEstruchBiology]

😁😁 thanks

[@MatT-vf7qo]

Where do the H+ ions come from?

[@MissEstruchBiology]

These are already within the cell from other molecules, most likely water that has been split.

[@GhostHawk272]

In the June 2022 Paper 1 Q9.1, the Mark Scheme says exactly the words: "Sucrose actively transported into phloem (cell)" as the first mark point. However, here you are saying that the sucrose moves into the companion cell by facilitated diffusion (6:30 in the video). So there is a difference here with a recent Exam paper Mark Scheme. Please could you clarify this point? Thanks

[@MissEstruchBiology]

this video goes through the exact stages (facilitated diffusion and then co-transport which involves active transport). The mark scheme has shortened this to just active transport

[@GhostHawk272]

@@MissEstruchBiology ok thank you!

[@anaisp8622]

So the hydrogen ions move back into the companion cells after going to the cell walls? I thought that the hydrogen ions were co-transported with glucose into the sieve tube elements.

[@MissEstruchBiology]

Hello, I hope this helps clarify :) At 7.15mins this is where I go through the process, and I state as you have said above. Here is the information from that part: Photosynthesis occurring in the chloroplasts of leaves creates organic substances, e.g. sucrose. This creates a high concentration of sucrose at the site of production, therefore sucrose diffuses down its concentration gradient into the companion cell via facilitated diffusion. Active transport of H+ occurs from the companion cell into the spaces within the cell walls using energy. This creates a concentration gradient and therefore the H+ move down their concentration gradient via carrier proteins into the sieve tube elements. Co-transport of sucrose with the H+ ions occurs via protein co-transporters to transport the sucrose into sieve tube element.

[@naaokailey7030]

@@MissEstruchBiology thank you, miss :)

[@anaisp8622]

@@MissEstruchBiology Thank you :)

[@erenchumihumtsoe2114]

Hi, I actually need you help! My lecturer has assigned us a project based on pressure flow model. Practical. So we’ll have to build something to explain the features of pfm but im unable to find any ideas. Il be so grateful if you could share me any ideas!

[@MissEstruchBiology]

Try on PracticalBiology or SAPS website maybe? Maybe something with straws and two mugs of water might work? Good luck

[@erenchumihumtsoe2114]

@@MissEstruchBiology right! Thank you for much for the ideas. Really:)

[@JM-mz1qd]

In the text book it says that H ions move with the sucrose molecules via a co transporter protein into the sieve tube elements but your saying that the H ions move back into the companion cells by diffusion. This confuses me as you shown in another video that co transporter protein moves two substances in the same direction (ilium video)

[@MissEstruchBiology]

Hello, there are multiple stages to this. Firstly, H+ is actively transported from the companion cells into the spaces within the cell walls. The H+ then diffuses through carrier proteins into the sieve tube elements. This enables the co-transport to occur. Sucrose is transported along with the H+ ions. Co-transport can be moving two molecules in opposite directions or the same direction, it depends on which co-transport protein they move through (think unique 3D shapes of proteins). I hope that helps :)

[@AM-hn7gj]

For point 2 at 9:24 is it supposed to say water potential increases?

[@dana-wc8ng]

That's what I was thinking as well. I'm gonna assume she made a mistake there cause water potential decreases as solute is actively transported out of it makes no sense.

[@MissEstruchBiology]

Hello, I am talking about the sink cell, not the sieve tube element, when I say the water potential decrease. (the sucrose is moving into the sink cell, so the water potential decreases) Hope that helps. :)

[@footballkick88]

my teacher told us that sucrose is activley transported into companion cell however it says here that it is by diffusion? im confused as to whose correct

[@alaynahurooj1995]

Hydrogen ions are actively transported not the sucrose, that moves because its concentration increases so moves from high->low down its conc gradient. Trust me shes right just learn it :)

[@MissEstruchBiology]

Hello, The simplified answer is yes, it is an active process overall, but it isn't sucrose that is actively transported. This may be were the confusion is. It is the hydrogen ions that are actively transported to enable the sucrose to diffuse, but sometimes this gets simplified to just saying the movement of sucrose involves active transport. If you have the OUP textbook for A-level it is on page 188 :)

[@footballkick88]

@@MissEstruchBiology thanks alot

[@chloedingle9450]

Hi, I was wondering when the H+ ions move into the sieve tube elements via carrier proteins, could you also say channel proteins instead?

[@Bert5627]

I don't believe so, as channel proteins mainly allow for facilitated diffusion. Co-transport (is a form of active transport), i believe is solely done by protein carriers, making the two non-interchangeable.

[@LouisMoir-Barnetson]

Hi Miss Estruch. Trying to learn this rather complex theory! You explain it very well, but I think you are incorrect in your explanation at 4:52 (let me know otherwise). In relation to the mass-flow model presented, the solute potential in the tank of water is zero and the sink cell is slightly negative (assuming consumption of sugars)-the sink cell will develop a positive hydrostatic pressure due to the mass flow of solution from source cell (due to the hydrostatic pressure gradient). This will result in the pressurised mass flow of water out of the sink cell i.e. due to a pressure gradient, not solute potential gradient.

[@hannahm2028]

6:28

[@xkapoor1724]

FYI- water potential can never be positive--

[@MissEstruchBiology]

yep that's true, but the phrase 'more positive ' is accepted as it means closer to zero

[@Heyjudeeeee12321]

co transporter made no sense to me

[@PabloEscobar-mm2xn]

Your leng